Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Generally, small-sized mobile devices use one or several cells for each device. On the other hand, medium- or large-sized devices, such as vehicles, use a medium- or large-sized battery system having a plurality of cells electrically connected with each other because high output and large capacity is necessary for the medium- or large-sized devices.
Pouch-shaped lithium-ion polymer secondary cells, which are usually used as unit cells of a medium- or large-sized battery system, have low mechanical strength. For this reason, a plurality of unit cells are mounted in a cartridge such that the unit cells are electrically connected in series or in parallel with each other, and a plurality of cartridges are electrically connected with each other to manufacture a medium- or large-sized battery system.
However, the cartridges are formed approximately in the shape of a plate, and therefore, the cartridges occupy a large amount of area. In addition, the distance between the cartridges is large to effectively dissipate heat generated from the unit cells, and therefore, the total volume of the battery system is increased.
Also, the protruding areas of a cathode and an anode of each unit cell are large, and therefore, an engineer who manufactures the battery system or a user may be exposed to a high risk of electrical short-circuits.
Furthermore, the electrical connection (in series or in parallel) between the electrodes is poor, and therefore, short-circuits may occur due to external forces, such as vibrations and impacts.